A standard depth key set utilized by a locksmith in association with a basic key cutting machine only guides the cutter with respect to the longitudinal position along the key blank for the selected cut. The rearward facing slope of each cut, that is the slope of the cut facing toward the bow of the key is made by the cutter shape itself. However, the cutter is not shaped to make a forward or tip facing slope. The forward face of the cutter is arranged at right angles to the longitudinal axis of the key. Thus a key, after cutting on the key machine, will include a series of cuts with one sloped and one perpendicular face. The locksmith must then use a hand file to produce a slope on the rear face of each cut. This slope must be made cautiously to avoid destroying the base of an adjacent cut or to cutting of the shoulder of the key blank. If the perpendicular faces are not filed to the proper slope the key will not operate the tumblers in the lock and therefore cannot be inserted properly into the lock. On the other hand, if the cuts extend too far and destroy the base of an adjacent cut the tumbler for that cut will not be stopped at the proper position and the lock will not operate. Should the filing destroy the key shoulder, then the key can be inserted too far into the lock and again will not operate the lock properly.
Micrometer cutters are available to custom cut each cut in the key to precisely correct depth and shape. However, such cutters are awkward to use especially in the field and are expensive to obtain and operate.
The deficiencies of conventional depth key sets are particularly apparent when it is desired to produce a set of master keys for a particular key type. Master keys and master key sets are frequently required by a locksmith operating in the field. Such key sets may, for example, be necessary so that a locksmith can carry all of the keys necessary to operate the locks of a particular foreign vehicle, so that he can answer a call where such a vehicle has been locked with the key inside. A similar requirement exists to have a master key for an apartment hotel building which will operate the door lock in all of the rooms in the apartment hotel while at the same time none of the individual door keys will operate any of the other door locks. Such master keys and key sets conventionally produced with special depth key sets utilize base depth that are precisely half-way between standard depths. For example, for a particular key blank a number six cut depth may have a blank base to cut base depth of 0.230 inches whereas a number five depth cut will have a depth of 0.260 inches. A master key with a cut depth of 0.245 inches (half-way between the number and number six cut depth) will operate locks set for either five or six depth.
It will be understood that a master key half cut between the depth at each cut position will operate a number of keys equal to 2 raised to a power corresponding to the number of pin tumbler positions. Accordingly, for a 5 pin tumbler lock with 5 cut positions along the longitudinal axis of the key will operate 25 or 32 locks.
The tolerance of locks for accepting keys with off specification depths is limited and it will therefore be apparent that precise control over the cut depth is required. For this reason it is very difficult to manufacture a half cut depth key set that will operate in conjunction with a conventional basic key cutting machine and produce repeatable results in developing master keys.
Accordingly, it is desirable to have a depth key set that may be utilized in conjunction with a basic key cutting machine to standard and master keys with precisely made cuts. Such a depth key set is particularly desirable where it provides guidance for the cutter on both slopes of a cut without interfering with an adjacent cut base or with the key shoulder.